I have a 1980 F250 and the door sticker says the stock tire size is 7.50-16. What is this in a radial size? I am wanting to put 245/75-16 on it. Is this close enough to the original tire to not cause speedometer problems and still look like they fit the truck? It is just 2wd and I am not interested in going extremely wide and/or tall.

 

The old 7.50-16 is appx a 32 inch tire. Those were the old standard bias ply tube truck tires. They were on split rims on the older trucks like mine. The 245/75-16 is 30.46 inches dia. Too small really if you want to keep the same dia. To use a 75 aspect ratio, you will need a 275 or 265/75-16 to have the same appx dia. "32.24" for the 275. A 265/75/16 is 31.64 dia. It's close enough to work fairly well, but a little bit smaller. Probably not enough to be a big deal.
Myself, I just got a new round of tires in June, and did the same switch. I dumped to split rims for solid wheels, and went with 235/85-16's. They are 31.72 dia. Pretty close to 32''...A 245/85-16 is 32.39''. It would be ok too. At the time I switched, I wasn't exactly sure what the dia of the 750-16's were. From measuring them, I got about 31.5 or so, but I had tread wear... I just ran across a thing that said they were 32 inches. I thought about a 75 aspect, but the price was a bit more, and I was kinda tight at the moment so went with the 85's. The 235/85-16 did give me a E load rating, which I needed on F-250. When I switched from the 32'' to 31.7'', I couldn't tell any difference in rpm vs mph that I could tell on the freeway. They seem the same. So a 1/2 inch ain't too much to worry about. MK